LAYOUT.md: single-draw design with an O(1) move chain; IRIS.md for notable API changes

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

IRIS.md

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+# iris: notable public API changes
+
+For Iris to read on her own time. Each entry is a change to iris's public
+surface that a widget author or app author would notice: a trait method
+added, removed or re-shaped; a type that callers construct differently; a
+capability that moved. Small and trivial changes do not go here.
+
+An entry gives the date, what changed, why, and a short before/after where
+it helps judge the change without the session that made it. Newest first.
+
+_No entries yet._